1. Field of the Invention
This invention relates to an apparatus and method for controlling the combustion of acid gas containing hydrogen sulfide in sulfur recovery units (Claus plants).
2. Prior Art
Sulfur is present in natural gas principally as hydrogen sulfide H2S and in other fossil fuels as sulfur-containing compounds which are converted to H2S during processing. The H2S is removed from the natural gas or refinery gas by means of one of the gas treating processes. The resulting H2S-containing acid gas is processed to recover sulfur. The recovery of free sulfur from gaseous streams containing hydrogen sulfide has become a valuable procedure in the petroleum gas industries. The Claus process is widely used for sulfur recovery from H2S. Conventional Claus plant consists of a thermal conversion section, and a few stages of catalytic conversion section, in series. Acid gas feed entering sulfur recovery unit consists of H2S and other uncombustible gases (nitrogen, CO2) and sometimes, in small amounts, combustible gases. The combustion in the thermal section is controlled by adding a controlled amount of air, required for burning one-third of the H2S to react with oxygen to produce SO2. The balance of the conversion is achieved in the presence of catalyst in the catalytic conversion stages provided via the reaction of two-thirds of H2S and SO2, to produce sulfur and water. Liquid sulfur is then collected in sulfur concentrators. However, not all the amounts of H2S and SO2 react. Some residual amounts remain in a tail gas. Very strict requirements to the residual H2S and SO2 make the control of the Claus reaction a difficult problem. Unlike the conventional combustion process, which allows for the use of different fuel-air ratios, the Claus reaction requires the stoichiometric values of H2S and air. Most commonly, the residual H2S is further burned and converted into environmentally less harmful SO2 and the latter is emitted. For that reason, excess of either H2S or SO2 compared to the stoichiometric values increases emissions, and only optimal H2S to SO2 ratio (corresponding to stoichiometric combustion), which is achieved by proper air-to-acid gas ratio, provides minimal SO2 emissions. Conventional control of the Claus reaction includes an air-to-acid gas ratio controller that generates a command for a main air flow controller, which manipulates a main air flow valve, and an analyzer controller of proportional-integral-derivative (PID) type that generates a command for a trim air flow controller, which manipulates a trim air flow valve. The set point (ratio value) for the ratio controller is entered by an operator. The analyzer controller uses the measurements of residual H2S and SO2 in a tail gas to generate a command for the trim air flow controller, so that it generates a command to bring tail gas H2S-to-SO2 ratio to the set point 2. This control scheme may provide a satisfactory performance of the control system if the acid gas flow is relatively steady. If the acid gas flow fluctuates (which is normally the case) it becomes very difficult to achieve a satisfactory performance of the control. As a result, in many cases a very expensive additional treatment of the tail gas aimed at removing the residual H2S and SO2 may be needed to reduce emissions.
U.S. Pat. No. 3,985,864 (1976) of Lucien H. Vautrain, et al. discloses an automatic control system for a Claus sulfur plant. The flow rate of the oxygen-containing gas to a process for the oxidation of hydrogen sulfide is regulated so as to be responsive to changes in pressure in the hydrogen sulfide feedstream. In both patents, the overall ratios of oxygen to hydrogen sulfide are adjusted to maintain the desired ratio of hydrogen sulfide to oxygen feed. In carrying out stoichiometric control of the hydrogen sulfide gas stream and oxygen-containing gas stream, there are five objectives cited. These objectives are (1) maintain the quantity of oxygen below that stoichiometrically required for the oxidation of the hydrogen sulfide in order to prevent the formation of sulfates; (2) maintain the oxygen quantity as close as possible to the stoichiometry required in order to promote the highest possible efficiency of oxidizing the hydrogen sulfide-containing gas stream and to reduce the sulfur content of the gaseous effluent from the process; (3) maintain stable control of the process while achieving the above two objectives, even though the gas flow may vary; (4) maintain stable control, even though the hydrogen sulfide content of the hydrogen sulfide gas-containing stream may vary; and (5) effect stable control of the process while achieving the above four objectives, even though there is a time between the occurrence of a variation in one or both of the process feedstreams and the occurrence of the measurement of the effect of that variation on the gaseous effluent from the process. In summary, both patents disclose an automated flow control scheme to maintain the required stoichiometry of the Claus reaction.
U.S. Pat. No. RE 28,864 of Andral, et al. (with a foreign priority date, application No. 70.45812 in France) discloses process and apparatus for automated regulation of sulfur production units. The process incorporates oxidation of hydrogen sulfide, in which the flow of gas carrying oxygen into the unit is regulated so as to keep an operating parameter, based on measurement of the sulfurous compound of the residual gases, level with a reference value. It is characterized by the fact that the control signal, used to regulate the flow of gas containing oxygen at the unit inlet, is a combination of a signal based on measurements taken at the inlet, and representing the theoretical flow of this gas needed to keep the operating parameter at its reference level and another signal representing the correction needed in this flow to adjust the instantaneous value of the parameter to the reference level. The disclosed process claims better control of the sulfur unit, with increased efficiency and reduced atmospheric pollution.
U.S. Pat. No. 4,100,266 of Smith (1978) discloses an automatic control system for a Claus sulfur plant, in which control of a process is accomplished by manipulating the flow rate of a feed stream containing oxygen to a furnace in such a manner that a desired proportion of the hydrogen sulfide fed to the furnace is converted to sulfur dioxide. The flow rate of a feed stream containing hydrogen sulfide to a tail gas cleanup process is also manipulated utilizing feedforward and feedback control to maintain the hydrogen sulfide and sulfur dioxide concentrations in the gas stream from the tail gas cleanup process at acceptable levels. Some other variations of the described principle were disclosed in U.S. Pat. No. 5,965,100 of Khanmamedov (1999), and 7,754,471 of Chen (2010). The described control principle may provide a satisfactory performance of the control system if the acid gas flow to the sulfur recovery process is a relatively constant value. If the acid gas flow fluctuates (which is normally the case) it becomes very difficult to achieve a satisfactory performance of the control. As a result, in many cases a very expensive additional treatment of the tail gas aimed at removing the residual H2S and SO2 is normally needed. Control performance has a significant effect on the emissions of environmentally harmful substances, which can be substantially mitigated by the disclosed adaptive ratio control.
U.S. Pat. No. 5,176,896 of Bela discloses apparatus and method for generation of control signal for Claus process optimization. It incorporates generation of a control signal for the optimization of sulfur removal in a Claus process unit that comprises oxidizing a portion of the tail gas stream exiting the Claus unit by contacting a portion of the tail gas with an oxygen-containing gas in the presence of a catalyst which oxidizes H2S to SO2, measuring the temperature rise associated with the oxidation reaction, converting the measurement to an appropriate control signal, and using the signal to control the rate of air flow into the Claus unit. Canadian Pat. No. CA 1323173 to Lagas et al. discloses a process for the recovery of sulfur from a hydrogen sulfide containing gas (acid gas), which comprises oxidizing hydrogen sulfide with oxygen, and then reacting the product gas of this oxidation further by using at least two catalytic stages, in accordance with the equation: 2H2S+SO2=2H2O+3/n Sn. In order to improve the process and the process control, the invention is characterized in that the H2S concentration in the gas leaving the last catalytic stage is controlled to have a value ranging between 0.8 and 3% by volume by reducing the quantity of combustion or oxidation air passed to the oxidation stage and/or causing a portion of the hydrogen sulfide containing feedstock gas to bypass the oxidation stage and to be added to the gas flowing to a catalytic stage. As described, typical control of the Claus reaction includes an air-to-acid gas ratio controller that uses measurements of acid gas flow and generates a command for a main air flow controller, which in turn manipulates a main air flow valve, and an analyzer controller of proportional-integral-derivative (PID) type that uses measurements of H2S and SO2 in a tail gas and generates a command for a trim air flow controller, which in turn manipulates a trim air flow valve. The main drawback of the available controls is related to possible fluctuations of acid gas flow and slow response of the tail gas concentrations to changes in a tail gas flow and air flow. If a tail gas flow changes the main air flow controller responds to this change very quickly incrementing air flow. However, the air-to-acid gas flow ratio demand is entered by an operator and is not optimal, so that the air flow increment would not fully correspond to the acid gas flow increment, and the increment of air flow will be either smaller or larger than the optimal necessary for a stoichiometric combustion. As a result, after all the reactions occur the concentrations of H2S and SO2 in a tail gas will change. Yet, it will only be measured with some delay, after this reaction has already happen, which results in insufficiently high quality of control, observed as high fluctuations in a tail gas H2S-to-SO2 ratio. Another drawback is related to uncoordinated motion of the two air valves, so that one valve may have a command to open, thus increasing air flow, and the other valve to have the command to close, thus decreasing air flow, while in fact no change may be required in terms of total air required. This uncoordinated motion of the two air valves contributes to the deterioration of the control performance, as the valves respond to their commands not instantaneously but with some lag, which differs between the two valves. Those lags result in the deviations of the total air flow from the total air flow demand (sum of the two demands) and overall performance deterioration.
It would be desirable to calculate and use an optimal value for the air-to-acid gas ratio demand, so that any fluctuation in an acid gas flow should be immediately matched by corresponding amount of air-through the action of the ratio controller.